1. Field of the Invention
The invention generally relates to electrical motor drive systems and in particular to a stability control circuit for use with an electrical induction motor.
2. Description of Related Art
Electrically powered motors are being developed and refined, particularly for use in powering automobiles. A typical drive system for an electrically powered vehicle includes a storage battery connected to one or more multiple phase induction motors through a drive control circuit. Three phase induction motors are typically used. The drive control circuit converts DC power from the battery to three-phase power signals for energizing the windings of the three phase induction motor or motors. The drive system converts electrical power to mechanical power subject to feedback control mechanisms designed to insure an adequate amount of torque at all operating vehicle speeds when the vehicle is in a "motoring" mode. A typical drive system is also capable of operating in a "regeneration" mode where the motor operates as a generator to convert mechanical energy of the moving vehicle into electrical power that may be used to recharge the on-board batteries.
Referring to FIG. 1, a typical prior art drive control circuit for use with a three phase induction motor is considered in more detail. The control circuit, shown generally as 100, receives an input phase reference signal .theta. on line 120. The phase reference signal is developed from the sum of a tachometer phase signal and a slip phase signal. The phase reference signal is asserted as an input to reference generators 101 and 102. The output of reference generator 101 on line 128 is a phase A reference signal; f.sub.1, conforming to the following relationship: EQU f.sub.1 =k.sub.1 sin (k.sub.2 .theta.)
where k.sub.1 and k.sub.2 are constants. The output of reference generator 102 on line 130 is a phase B reference signal, f.sub.2, conforming to the following relationship: EQU f.sub.2 =k.sub.1 sin (k.sub.2 .theta.+2.pi./3).
It will be observed that the phase B reference signal corresponds to the phase A reference signal advanced by 120.degree..
The phase A reference signal on line 128 is asserted at the non-inverting input 108 of differential amplifier 109. The inverting input of amplifier 109 is coupled to a current sensor 108 on the phase A motor winding. Amplifier 109 compares the current sensed in the phase A winding with the phase A reference signal to generate an output error signal. In like manner, amplifier 111 compares the current sensed in the phase B motor winding with the phase B reference signal to generate a phase B error signal. A summing network and amplifier represented by block 112 generates a phase C error signal that is the negative sum of the phase A and phase B error signals. The three phase error signals are asserted at the non-inverting inputs of respective differential amplifiers 114, 115 and 116. The inverting inputs of each of these amplifiers is coupled to a triangular wave oscillator 113 operating at the switching frequency. The outputs of amplifiers 114, 115 and 116 are thus pulse width modulated signals where the percentage modulation is proportionate to the respective error signal. These outputs are then coupled to respective drivers (not shown) to control solid state switches that regulate the flow of current through the respective motor windings.
An exemplary prior art motor drive system, incorporating the conventional features described above, is described in U.S. Pat. No. 5,099,186 issued to Rippel and this inventor.
However, in conventional systems such as described above, stability problems have been observed under certain operating conditions, particularly when the motor is operating at high speed with a high torque command. Such instability has been determined to be inherent in conventional systems and is typically manifested by oscillations in the motor torque. Such lack of stability within conventional control circuits prevents effective control of the operation of the three-phase induction motors and results in non-optimal performance of the motor system.